The present invention relates to a continuous compounding and mixing apparatus which performs automatic measuring, compound, mixing, and feeding operations in the stage of mixing and feeding various materials to a polymer processor.
In processing a polymer, some addition agent, pigment, and at least one other kind of polymer are compounded and mixed with a polymer material in most cases. In these operations, the most frequent and technically difficult one in view of accuracy is the compounding and mixing of a pigment. Accordingly, at present, there is an extremely urgent demand in this field for equipment that is capable of performing automatic measuring, compounding, mixing, and feeding of each component with high accuracy in the stage preceding a polymer processor. Some proposals are already provided, but there exists no product functionally satisfactory.
Common features in the conventional feed systems in the field intended for compounding and mixing of pigment with a relatively high accuracy are that a major component material and a minor component material are measured and fed by a batch method and that a mixer is also based on the batch method. That is to say, according to the technical concept common in the conventional systems, since the pigment is extremely small in quantity as compared to the polymer, the pigment quantity in one automatic measurement is increased up to the absolute quantity wherein the required measuring accuracy is attainable, and further the polymer with which the pigment is to be mixed is measured by the batch method as one lot of a large amount corresponding to said absolute quantity, and then the polymer and the pigment are combined.
For example, under the requirements that the minimum concentration of pigment against polymer is 0.1 percent and the mixing accuracy is within .+-.1 percent thereof, and that an automatic pigment measuring equipment is capable of measuring with the accuracy of .+-.0.1 gram, then it is necessary to measure at least 10 grams as the quantity of pigment corresponding to the measuring accuracy of .+-. percent. Since the polymer quantity 10 kilograms corresponds to the concentration 0.1 percent of pigment 10 grams, under such conditions, the desired accuracy is supposed to be obtained by regarding 1.0 kilogram of polymer as one batch quantity in measurement.
In the present state, automation is still incomplete, and most operations are performed manually (particularly in fields where a high accuracy is required). Also, in conventional automated systems, with the same intention to improve the measuring accuracy by enlarging the measuring size, amounts ranging from 100 to 200 grams are measured by the batch method and are mixed by means of a tumbler mixer based on the batch principle. In other words, in the conventional automation system, the attempt is merely to mechanize the system with respect in the conventional manual operation.
Accordingly, in such conventional automation systems, there occurs the defect that the mixing accuracy of concentration is low in practical function. That is to say, since a system is installed for each polymer processor, as viewed from installation cost and operational reason, it is impossible to employ a large batch quantity above 100 kilograms as in the manually operated tumbler mixer. One batch quantity in the conventional automatic mixer ranges from 5 to 10 kilograms. In the case of 10 kilograms, the quantity corresponding to the present concentration 0.1 to 2.0 percent is from 10 to 200 grams, so that a measuring accuracy of .+-.0.10 gram is required for attaining the accuracy of .+-.1 percent. Considering the physical property of pigment and the conditions of allowable installation cost, it will be almost impossible to realize automatic measuring equipment that can perform measurement up to a maximum of 200 grams and yet with the high accuracy of .+-.0.10 gram. Consequently, in a typical example based on the conventional method, a concentration error of .+-.5 percent is indicated in the use of the pigment concentration of 1.0 percent.
The reason for the difficulty in attaining a satisfactory automation system as described above is attributed to the following causes:
1. The pigment being a smaller mixture component (minor component) is often of viscous property and thereby causes bridging. That is to say, the powder is liable to get stuck, so that handling for measurement in a measuring equipment is difficult.
2. The required variation range for the mixing concentration of pigment against polymer is so wide as to cover from 0.005 to 8 percent, and mixing should be possible at least in a range from 0.1 to 2.0 percent. Furthermore, the accuracy of the pigment component is required to be .+-.2 percent, preferably .+-.1 percent, against the concentration.
3. Since the industrial unit of a processing installation system is remarkably small as compared with a chemical plant system, a high-class automation system is not usable, and the allowable investment amount for equipment is at most from about a half million yen to 11/2 million yen. Accordingly, the allowable installation cost is excessively low in view of the strict requirement that the minor component of the mixture should be measured automatically in a wide mixing range and yet with a high accuracy through the entire range, notwithstanding its difficulty of handling.
4. Change of the kind of pigment (hereinafter referred to as color change) is so frequent that, unless the installation is constructed in an outstanding simplicity, the loss in working time or material when the apparatus is cleaned for a color change causes great economical effects.
In the case where a large quantity of fluid is handled in a chemical process plant, a continuous agitator is used as an efficient mixer. However, when the quantity to be processed in small with the minor component having little in quantity and yet a high accuracy is required, measuring and mixing based on the batch method are often employed even for liquid materials. Therefore, it has never been recognized heretofore that amelioration of the defects existing in the conventional method is attained at a cost lower than in the conventional method by the use of a continuous mixing vessel for the automatic measuring and mixing of polymer and pigment intended to be mounted to each polymer processor which is on a small industrial scale and presents so difficult a problem in view of accuracy that the required accuracy cannot be attained even by the batch type mixer. That is to say, the reason that the continuous mixing vessel serving as an automatic pigment measuring-mixing equipment to be mounted to each processor in the polymer processing industry has not been noted heretofore as viewed from practical utilization is attributed to the following causes:
a. When a high accuracy is required with respect to compounding ratio, in the case of a continuous mixing vessel to which materials should be fed substantially continuously, it has been affirmed that there is a great difficulty in performing measurement control by reading out the respective flow rate of polymer and pigment at the accuracy of .+-.1 percent or so. PA1 b. In the stage of mixing the pigment and the polymer, a considerably complete mixing is required. And with the entire pigment adhering to the surfaces of polymer particles, a homogeneous state should be attained. For this reason, the pigment and the polymer are mixed for about an hour by a tumbler mixer according to the conventional method, or batch type mixing is performed according to the latest method for about 50 seconds by a high-speed mixer at the speed of 1500 to 2000 r.p.m.
In other words, generally there has been a previously accepted vague concept that complete mixing to meet the requirements of the above-described accuracy could not be achieved by a continuous mixer, and consequently an attempt to put the continuous mixer into practical use has been neglected.
On the other hand, for the purpose of attaining some agitation effects in a processor hopper itself, a rotating type agitator was provided inside the hopper on rare occasions. However, the object of such application was chiefly to suppress the separation of different kinds of materials while expecting merely supplementary agitation effects, differing from the object of this invention that mixes a plurality of materials for the first time in the processor hopper at a high-accuracy compounding ratio and thereby achieves complete mixing effects.
Now, the definition or essence of a continuous mixer will be analyzed. In a batch type mixer, a batch cycle operation is performed in such a manner that materials not mixed at all (hereinafter referred to as zero-mixed) are charged into an empty mixing vessel until reaching the effective capacity of the mixer, then mixing proceeds over a period of time, and finally the entire amount is discharged in the state where mixing has reached a required degree (hereinafter referred to as sufficiently mixed). The characteristics of this type of mixing process may be said to be that the mixing state changes with the lapse of time in every point in the mixing vessel and that the substance level in the mixing vessel repeats two states, i.e., a fully charged state and an empty state. In the continuous type mixer, its characteristics reside in that the mixing state at a certain point in the mixing vessel is kept substantially constant regardless of the lapse of time, and the substance level in the mixing vessel is always above a certain level so that there occurs no extremely great variation (hereinafter referred to as a substantially fixed level). In fact, the requisites for the continuous mixing vessel are that the substance level and the degree of mixing in the mixing vessel are to be substantially in steady state, but neither charge nor discharge need be completely continuous. On condition that the steady state average is maintained, intermittent charge or discharge causes no problem.
The object of this invention is to eliminate the defects in the above-described conventional feed system and to solve the problematical points in the process and apparatus, thereby providing a formerly unrecognized automatic system for measuring, compounding mixing, and feeding materials to a polymer processor on the basis of the analysis of the said continuous mixer.
In this invention, means taken for attaining a high-accuracy compounding ratio at relatively low cost are characterized as follows. As already mentioned, charging both polymer and pigment to the mixing vessel need not be continuous, and particularly the pigment of a minor quantity may be charged concentrically by a fixed amount (for example, 10 to 50 grams) at a certain point of time. It is convenient that this point of time is selected to be, for example, the moment when charging the polymer required for compounding the pigment has been terminated. Accordingly, it is important to grasp the polymer charging quantity not as an instantaneous value such as r.p.m. but as an integrated value with the required accuracy. In order to grasp the polymer charging quantity as an integrated value with a high accuracy, an exemplary embodiment of this invention adopts a method which digitally counts the number of feed units fed by an automatic measuring-feeding apparatus which discontinuously measures and feeds a fixed weight or capacity as a feed unit. A control circuit is so composed that when the amount counted by the digital counter has reached a preset value, a fixed amount of the pigment is fed. This can be carried into effect by providing a digital counter for the polymer side and a preset counter for setting where the pigment is to be fed. Moreover, it will also be possible to form the said control circuit in such configuration that, by the provision of digital counters and feed quantity preset counters for both pigment and polymer sides, the pigment and the polymer are counted independently of each other by the counters until they have reached the respective values previously set by the preset counters. The probable causes interrupting the acquisition of a complete mixture at the discharge outlet in the continuous mixer include that the zero-mixed materials, immediately after being charged, make a short pass directly to the discharge outlet so that an accurate stay time (mixing time) cannot be given to the entire charged materials, and also that there occurs separating action resulting from the centrifugal force or gravity of each material.
For the purpose of preventing such a short pass, this invention employs the following means to achieve a remarkably satisfactory result for a simple structure. The said means is to emphasize the piston flow character with respect to the travel of the materials in the mixing vessel and simultaneously to decrease the back flow caused by the agitator. In this invention, in order to naturally form a flow without opposing the natural flow of the substance in the mixing vessel due to gravity, the mixing vessel is shaped to be substantially vertical so as to effect charging in the upper portion and discharging from the lower portion.
In an attempt to prevent the occurrenece of back flow, this invention employs a rotary agitator whose rotating shaft is substantially vertical, meaning that the plane of rotation is substantially horizontal. Additionally, the shape of the agitator vanes (particularly at the level immediately below the material charging port) is selected to be that of a horizontal flat plate, avoiding vanes that have a vertical lead angle and thereby minimizing the occurrence of vertical discharging component force. Square rods, round rods, and elliptical rods without vertical pitch have also been selected.
The fact that the rotation plane of the agitator is horizontal means that the plane of rotation drawn by each agitation vane around the rotary shaft is horizontal, but it does not mean that the agitation vane per se should be constructed horizontally. In order to obtain an agitation vane having no angle of lead in the vertical direction, it is necessary for it not to have an angle of rake to a circle drawn by rotation of each part of the agitation vane in a broad plane such as in the case of a propeller. Accordingly, the agitation vanes are formed from angular rods, round rods, or the like. It is not always indispensable that they extend horizontally. However, they must be designed so that they produce no extrusion force in the vertical direction even though the material is bent or distorted in any manner whatever.
The separation trouble resulting from gravity or centrifugal force differs with the combination of material components to be mixed, so that a sweeping statement may not be given. The ordinary pigment has a sufficiently strong adhesion to the charged polymer so that, in mixing pigments of less than several percent against the polymer, it has been confirmed experimentally that the use of the above-described mixer is capable of completely eliminating the feared problem of separation.
In any special case other than the mixture of ordinary pigment and polymer wherein separation is liable to occur due to the absence of adhesion and/or wide difference is subsidence properties depending on gravity or grain size, the speed of rotation of the agitator may be made variable or so set as to be switched over to intermittent rotation in order to avoid separation resulting from the centrifugal force of the agitator. Next, for the purpose of obtaining at the outlet port of the continuous mixing vessel a desired mixing ration which is always uniform and fixed, this invention positively adopts the following means.